This invention relates generally to an apparatus and method utilized in the process of manufacturing semiconductor lasers. More particularly, the invention relates to an apparatus and method that permits batch processing of the semiconductor lasers so as to speed up the manufacturing process for semiconductor lasers, and also provide improved device characteristics and a longer lifetime for high power applications.
In recent years, semiconductor lasers have found a number of technological applications, including optical communications systems, optical storage systems, and laser printers. Semiconductor lasers suitable for these applications are generally formed by depositing a multilayer structure on a substrate so as to form a laser cell from which a large number of lasers can be made. The laser cell is typically cleaved along parallel planes into a number of laser bars. The laser bars are subsequently cleaved into a number of individual semiconductor dies, each of which may ultimately become a semiconductor laser diode.
Generally, after the laser cell is cleaved into laser bars, and prior to cleaving into individual dies, it is common to coat a surface of the laser bars with one or more of various layers of material. Such layers of material may be used, for example, to form passivation layers or mirror layers. It may be desirable to grow one or more of these layers by molecular beam epitaxy (MBE) or other epitaxial growth techniques, in order to produce uniform layers. However, epitaxial layer growth is a very slow process that is conducted in a specialized deposition chamber that is designed based upon the growth technique. Typical deposition rates are on the order of about 1 xcexcm/hr. Thus, the epitaxial growth of layers on each individual laser bar as they are separately provided within a deposition chamber results in prohibitively long processing times for the commercial manufacture of semiconductor lasers.
Methods for cleaving and coating semiconductor lasers are known. For example, U.S. Pat. No. 5,144,634 discloses a method for cleaving and passivating a semiconductor laser. A single laser bar comprising two scribe marks is mounted in a carrier. The laser bar is cleaved along the two scribe marks while in vacuum to form mirror surfaces which are then passivated by e-beam evaporation of a layer of amorphous silicon or the like. This process of individually cleaving and passivating the resulting facets is cumbersome, and is generally too time consuming for optimal commercial production methods.
In addition, many technological applications of semiconductor lasers require operation of the lasers at high power outputs (typically above 30 mW) for extended periods of time. The operation of semiconductor lasers at high power outputs may cause considerable dissipation of heat at the laser end surfaces, which over time can degrade these surfaces. This deterioration, termed catastrophic optical damage (COD) in the art, reduces-the stability and lifetime of the semiconductor laser. Thus, the maximum power at which a semiconductor laser can be operated for extended periods of time may be severely limited.
Applications of semiconductor lasers for communications systems require operation of the lasers at very high power outputs (typically above 100 mW) for extended periods of time. At these very high power outputs the potential for COD is significantly increased and greater care must be exercised in the formation of the laser end surfaces in order to reduce the possibility of COD.
One known way to reduce COD is through the use of xe2x80x9cwindow layersxe2x80x9d on the end surfaces of the semiconductor lasers. A window layer is a layer of material having a band gap that is significantly higher than the material forming the multilayer structure. The window layer is largely transparent to the laser light, and thus serves to lengthen the laser cavity. This reduces heat build-up on the end surfaces of the multilayer structure, and consequently reduces COD. Such window layers are typically on the order of 100-200 nm thick.
It is also known that these widow layers provide even greater protection against COD when deposited on xe2x80x9ccleanxe2x80x9d contamination-free laser end surfaces. Thus, laser bars are cleaved in ultra-high-vacuum (i.e. in the order of 1xc3x9710xe2x88x928 torr or better) to achieve contamination-free mirror surfaces, which mirror surfaces may then be provided with a window or passivation layer. Unfortunately, creating the ultra-high-vacuum environment necessary to create contamination-free laser end surfaces is also a slow process. Using methods well known to the art to create or pump down to such high levels of vacuum, i.e. better than 1xc3x9710xe2x88x929 torr with partial pressures of oxygen and water better than 1xc3x9710xe2x88x9212 torr, require at least 12 hours to achieve and typically require 24 hours or more. A method of mirror passivation where a single semiconductor bar is cleaved and passivated in a vacuum chamber while at ultra-high-vacuum is disclosed in U.S. Pat. No. 5,063,173 to Gasser et al.
In addition to protection against COD, several further characteristics are desirable. Power output and maximum device lifetime are critical criteria of semiconductor lasers, especially for use in communications systems. The formation of the window layers as a single crystal on laser end surfaces provides excellent protection against COD, while producing superior device characteristics (e.g., power output and stability). However, as explained above, creating ultra-high-vacuum levels for making contamination-free laser bar end surfaces and the subsequent growth of a single crystal layer on the end surfaces is necessarily a very slow total process. For example, the growth stage of a 100-200 nm single crystal film takes at least 30 minutes. One or more such layers may be desirable that may need to be separately grown on more than one mirror surface. This is in addition to the time required to achieve ultra-high-vacuum levels as noted above. This total time requirement essentially prohibits the growth of single crystal layers on individual semiconductor lasers in ultra-high-vacuum as a production tool.
Finally, the prior art passivation and window layers generally serve no usefulness as part of the mirror stack for the laser. Thus, after application of the passivation and/or window layers, it is still necessary to add multiple layers of amorphous material to each end surface so as to adjust the reflectivity of the surfaces.
The present invention overcomes the disadvantages and shortcomings of the prior art by providing an apparatus and method for batch processing a plurality of semiconductor laser bars allowing for simultaneous growth of layers of material on laser bar end surfaces. Batch processing a plurality of semiconductor laser bars not only speeds up the production process, but enables incorporation of the growth of single crystal layers on the laser end surfaces in production processes, yielding enhanced protection against COD and improved laser improved device characteristics.
Also, by the present invention, a method for producing semiconductor lasers is provided with improved protection against COD and superior device characteristics, including long lifetimes, high power outputs, and improved stability.
The present invention includes a method for batch processing one or more semiconductor laser cells, each comprising a multilayer structure formed on a substrate, in vacuum. The process includes the steps of cleaving a plurality of semiconductor laser bars from one laser cell and then depositing a layer of material on an end surface of each of the semiconductor laser bars simultaneously. Batch processing a plurality of semiconductor laser bars substantially reduces the average growth time per individual laser bar, thus making feasible the commercial processing of semiconductor lasers coated with end surface layers.
The method of the present invention is particularly useful in the formation of single crystal high band gap mirror layers on the laser end surfaces, wherein relatively thick mirror layers made of a single crystalline layer can be grown on a plurality of laser bars simultaneously. This permits the mass production of semiconductor lasers with improved device characteristics (e.g., higher power outputs and improved stability) and enhanced protection against COD relative to the conventional use of lasers comprising polycrystalline protective layers.
The present invention also includes an apparatus for batch processing semiconductor lasers. The apparatus includes a vacuum chamber and a pump which preferably reduces the pressure in the vacuum chamber to below 1xc3x9710xe2x88x928 Torr (that is, preferably to the level of ultra-high-vacuum). A support structure is provided within the vacuum chamber for supporting at least one laser cell thereon. A laser cell clamp is preferably mounted within the vacuum chamber to be positionable so as to hold a laser bar located along an edge of the laser cell at a desired cleaving position. A cleave bar that is operatively supported as well within the vacuum chamber is movable relative to the laser cell clamp, so that the motion of the cleave bar causes the laser bar located along the edge of the laser cell to be cleaved from the laser cell. After one cleaving operation, the laser cell clamp can be released to permit the laser cell to be repositioned so that a next laser bar position of the laser cell can be positioned for a next cleaving operation. Repositioning of the laser cell may be caused by gravity and or by the application of an additional force. Preferably, the support structure for the laser cell includes a stop to define the cleaving position of a laser bar portion. A cassette is preferably also provided which is operatively positioned to receive a plurality of laser bars after they are cleaved from the laser cell. The cassette also preferably positions and holds the laser bars such that at least one of the side surfaces of each of the laser bars is substantially exposed. A deposition source is then used to deposit a layer of material on the exposed side surface(s) of all of the laser bars simultaneously. More than one such layering process may be conducted as desired.
The cassette preferably holds the laser bars such that both side surfaces of the laser bars are substantially exposed. This may be accomplished by shallow slots on the cassette which retain edge portions of the laser bars while maintaining most of the side surfaces of the lasers bars exposed for depositing material thereon.
The support structure is preferably an inclined guide, which is inclined toward a cassette in position, so that the laser cell can be gravitationally indexed after each cleave. In its preferred operation, a laser cell clamp presses against the laser cell on the inclined guide, while the laser cleave bar presses against the laser bar portion that is located along the edge of the cell so as to cleave off the laser bar, which then falls gravitationally into the cassette. The laser cell clamp and cleave bar are then released, and gravity preferably causes the laser cell to move toward the cassette, so that the cleaving operation can be repeated.